This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
At the present time, facsimile transmitters and receivers are capable of operating in different transmission modes. For example, many facsimile transmitters and receivers in the United States facsimile market are capable of transmitting in 4 and 6-minute transmission modes. In order to signal the particular transmission mode to an unattended receiver such as the Model 410 manufactured by the Xerox Corporation, a signal is transmitted during the sync period from the transmitter having a frequency of approximately 1500 Hz. representing the synchronizing pulse for a 6-minute transmission mode and a signal of approximately 1250 Hz. is generated representing the synchronizing pulse for the 4-minute transmission mode. By providing a 15 second synchronizing period at the beginning of a transmission, it is possible for a receiver to detect the frequency of the synchronizing pulse and select the appropriate scanning rate before the document transmission begins. Typically the duration of the synchronizing pulse is constant and the occurrence of the pulse relative to the rotation of the facsimile drum clamp past the document photodetector is a function of the tolerance of the sync pulse generator.
In copending application Ser. No. 617,104 filed Sept. 26, 1975 and copending application Ser. No. 886,147 filed Mar. 13, 1978, both of which are assigned to the assignee of this invention, the synchronizing pulses are varied in frequency and duration as well as time lapse between synchronizing pulses to signal a plurality of transmission modes. This signalling is performed by the transmitter so as to advise the receiver of the type of mode being utilized in the transmission.
The Xerox 410 as well as Graphic Sciences DEX4300 also provide for signalling from a receiving unit to a transmitting unit to indicate a state of readiness to receive. Such a ready to receive signal which is oftentimes referred to as a handshake typically comprises a single frequency signal of relatively short duration which is transmitted from the receiver to the transmitter prior to the transmission of any information from the transmitter to the receiver. The function of the signal is to clearly indicate state of readiness of the receiver and there is no additional information provided by such as single frequency signal.
Where a transmitter is capable of different modes of transmission, some of which may be more desirable than others or particularly well suited to the document which forms the subject matter of the transmission, it becomes important for the transmitting unit to recognize the capability of the receiving unit. While this is oftentimes done by voice communication between operators at the transmitting unit and the receiving unit, such voice communication requires the full understanding of the operator at the receiving unit as to the capability of the receiving unit. Moreover, the communication can be time-consuming and prone to error especially when a receiving operator is required to preset the receiving unit to the correct combination of a number of possible operating modes. Of course where the receiving unit is an unattended machine, such voice communication is not possible.
Any signalling as to the capability of a receiving unit should be compatible with existing facsimile systems. In other words, ready to receive signals capable of indicating particular modes of operation at the receiver must distinguish over the ready to receive signals presently generated by certain existing facsimile equipment while at the same time distinguishing over the absence of any ready to receive signal which is characteristic of other facsimile equipment.
Any mode capability signalling the receiving unit must also be reliable. In this connection, the signalling must be noise immune so as to assure that the transmitting unit reliably detects the mode capability of the receiving unit. At the same time, it is desirable to provide this reliability without making the full bandwidth of the communication channel available to the transmitting unit for purposes of document transmission.